The field of the invention relates, in general, to network echo cancellers used in telephony systems, and more particularly to a method of detecting active echo cancellation.
Echoes, in general, are signals returning to their original source by traveling in the opposite direction of the original signals. Echoes arise in telephony systems when part of its corresponding speech signal energy reflects back from the impedance mismatches in analog parts of a telephone network. Echo Cancellers are signal-processing devices that measure and then remove echoes in a telephone connection, by producing the exact opposite of a sampled speech pattern. Hence, the echo cancellers are used to clear up these echoes on voice channels between two points on the telephone network. The telephone networks are typically configured as a 4 to 2 wire conversion for a subscriber interfacing with a public switch telephone network. This wire conversion from the two-wire to the four-wire segment is typically performed via a device called a hybrid.
A typical long-distance telephone circuit can be described as being two-wire in the subscriber loop to the local hybrid, four-wire over the long portion of the network to the distant hybrid, and then two-wire to the distance telephone. Hence although the use of hybrids facilitates long distance voice transmission, impedance mismatches at the hybrids may result in echoes. The voice signal from one end of the network is reflected off the distant hybrid (the hybrid closest to the other end of the network) in the telephone network back toward the first end causing the echo.
Network echo cancellers are thus used in the land-based telephone network to eliminate echoes caused by impedance mismatches at the hybrids and are typically located in the central office along with the hybrid. The echo canceller located closest to one end of the network is thus used to cancel the echo caused by the hybrid at the other end of the network. It is however, generally recommended that the echo canceller should be switched off for connections carrying data traffic.
Known signal-processing methods have been developed for selectively removing or for substantially reducing echo signals. These methods produce a substantially accurate estimate of the echo signal that is then injected into the medium as a canceling mirror image of the echo signal. Inaccuracies due to such signal processing operations create signal residues, which end up corrupting the integrity of the original signal. Such distortions of the original signal are usually acceptable in voice communications. However if applied to a data line, the results are an inoperable link, as the data has been modified from its original form. This is especially a problem on lines that use voice on some channels and data on others. To prevent such data distortion, a need therefore arises to check if echo cancellers are operable on a line, or on an individual channel. Previous solutions were basically implemented by inquiring in person as to the presence of echo cancellers. Thus there is a need in the art to automate this detection of echo cancellers.